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November 2005
The LETGS is primarily used in conjunction with the spectroscopic array, the HRC-S. The HRC-S consists of 3 MCPs, with 1513 wires along the dispersion direction (the V-axis) and 121 wires along the cross-dispersion direction (the U-axis) with a wire pitch of 0.2057 mm. An amplifier taps into this crossed grid at every eighth wire, with the result that there are 190 and 16 ``taps'' along the V and U axes respectively. The charge cloud generated by an incoming photon at the base of the MCP is read out by these amplifiers, and the position of the event is determined using the so-called ``three-tap algorithm''. The tap with the strongest signal is designated as the site of the event, and the position is further refined by combining the signal from this amplifier, say Ai, with those from the adjacent taps to determine the fine position
fp = (Ai+1-Ai-1) / (Ai+1+Ai+Ai-1)However, because charge in taps beyond the nearest ones is uncollected, this results in gaps near the edge of the taps where there will be a deficit of events. Note that unlike telescope vignetting, pileup, or Quantum Efficiency (QE), this is not lossy (i.e., the photons are not lost; they are simply mispositioned) and can be deterministically corrected with a suitably constructed degapping algorithm.
The degapping algorithm used for Chandra data analysis was derived by fitting symmetric 5th-order polynomials around tap center, to lab data. The polynomial coefficients were stored in a FITS file in the CALDB and were used by the CIAO tool hrc_process_events to correct the RAW positions of the photons. The degap solution obtained from these polynomial fits still had some defects, such as a 1-pixel drop-off between taps, invalid corrections due to the assumed symmetry of the degapping, etc. These errors contribute to the observed non-linearities in the LETGS+HRC-S dispersion relation. In order to provide flexibility in updating the degap, the storage mechanism in the CALDB was changed to that of a lookup table where the required correction is tabulated as a function of the AMP_SF value for each RAW coordinate position (see Juda 2005).
The HRC-S poses some unique practical difficulties in the determination of degapping solutions using on-orbit data. Due to an uncorrectable error in the onboard electronics, the anticoincidence counter has been turned ineffective, and the telemetered data contain a large number of particle events, which strongly affects the pattern of the amplifier signals, and adversely affects the degapping solution. These background events are reduced, but never entirely eliminated, using various filtering steps during analysis. Therefore, degapping solutions are strongly tied to the filtering applied to the data. Further, the analysis naturally depends on photons dispersed by the LETGS, whose numbers depend on the shape of the source spectrum and the telescope effective area, and hence there are less data available on the outer plates. It is imperative that the true distribution of the photons across a tap be known exactly, or else the degap solution will be subject to large systematic errors.
We have derived corrections
to the wavelength scale based on analysis of sources with strong line
emission (Chung et al. 2005). This was first made available via a Perl script
corrlam
and has now been incorporated directly into the degap solution
(Kashyap
et al. 2005).
The new degap, which is the polynomial solution modified
by corrections derived from line-emission data, is available
from CALDB 3.2.0 onwards in the file
$CALDB/data/chandra/hrc/bcf/gaplookup/hrcsD1999-07-22gaplookupN0002.fits
This solution improves the performance of the LETGS significantly,
but does not account fully for all the systematic errors
in the dispersion relation. The rms deviation of the wavelength
differences drop from 0.014 Å (0.010 Å over just the
central plate) prior to the correction, to 0.010 Å (0.006 Å
over the central plate).
We have also developed an empirical method to determine the degap coefficients from continuum sources (Kashyap et al. 2003, 2004). This solution is undergoing testing and modification to handle the dispersion derived from line emission as above, prior to being made available for general use.
The following is a list of memos and papers (in chronological order) on the HRC event positions and degapping solution in general, and on the LETGS+HRC-S degapping and dispersion non-linearities in particular.
Last modified: 02/17/12
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